This invention relates to diode detectors and more particularly to a diode detector that is used for regulating the level of a radio-frequency (RF) signal and that is stabilized against variations of temperature and other parameters. Such a diode detector may be included in a radio telephone for regulating the output power of the telephone's transmitter.
Regulator loops having stabilized or compensated diode detectors are used in many applications where the alternating current (a.c.) output signal from an amplifier or other component is monitored to ensure a correct level. One exemplary application is a radio telephone, where the RF output signal from the telephone transmitter's power amplifier is monitored to ensure that it has the correct transmission power level. Temperature-compensated detectors are frequently used for this purpose because the response of a diode detector usually varies with temperature to an unacceptable extent. This is particularly so for portable devices like radio telephones, which can experience extremes of both heat and cold and yet must maintain well-defined output power levels to avoid interfering with other devices.
Diode detectors are also useful in other parts of a radio telephone network. In a typical cellular radio telephone communication system, transceivers are located at multiple fixed sites throughout a geographic area for providing radio communication in their respective surrounding coverage areas, which are usually called cells. Each fixed site transceiver provides an interface between the public switched telephone network and portable radio telephones and other remote terminals located in its cell. The fixed site transceivers and radio telephones communicate by exchanging RF signals, employing various modes of operation (analog, digital, and hybrids) and access techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and hybrids of these techniques.
In current cellular communication systems, communication channels are implemented by frequency-modulating RF carrier signals that have frequencies near 800 megahertz (MHz), 900 MHz, 1800 MHz, and/or 1900 MHz. Other current communication systems operate at other frequencies in the RF portion of the electromagnetic spectrum, such as the "Bluetooth" system at 2400 MHz. The output signals from amplifiers in such systems, for example the amplifiers in a cellular system's fixed sites, may also be monitored to ensure that they have the correct signal level.
One approach to meeting the power control requirements of these communication systems is to provide a stable RF signal detecting circuit that is not susceptible to variations in temperature and power supply voltage. Accordingly, there is a need for an improved signal detecting circuit that is highly stable and linear over a wide range of variations in both temperature and power supply voltage. Many other applications would also benefit from such an improved signal detecting circuit.
U.S. Pat. No. 4,523,155 discloses a temperature compensated diode detector, where a detector diode is incorporated in a half-wave rectifier. The compensator diode has a double function, as it partly compensates for the temperature dependence of the detector diode, and partly forms part of a bias circuit, whereby the voltage on the detector diode input is raised to a value corresponding to the voltage drop across the detector diode when it is conductive. The voltage on the detector diode output corresponds to the RF signal when it is positive, and otherwise assumes the value zero.
European Patent Publication No. EP 0 834 987 describes a regulator loop for controlling the RF output power from a power amplifier of a radio telephone that has a diode detector for detecting an RF signal from the power amplifier and for providing an output signal in response thereto, a controlling circuit for generating a control signal for the power amplifier that depends on the output signal from the diode detector and a reference signal representing the desired output power of the power amplifier. A portion of such a transmitter is shown in FIG. 1.
An RF signal is provided at an input of a power amplifier 3, in which the signal is amplified to achieve the desired transmission power level at the amplifier's output. A directional coupler 4 disposed at the output of the power amplifier 3 generates, via an inductive coupling, a loop signal representing the output signal of the power amplifier 3. The loop signal is fed to a diode detector 5 (shown schematically as a single diode), which generates a d.c. output signal that is proportional to the loop signal and thus the output of the power amplifier. The output signal of the diode detector 5 and a power control signal are summed up by two resistors R.sub.10 and R.sub.11, thereby generating an error signal that is amplified by a loop amplifier 6, the output of which controls the gain of the power amplifier 3.
According to EP 0 834 987, the diode detector 5 is a voltage divider configuration forming part of a d.c. path across which a d.c. bias is applied. The voltage divider configuration is a serial connection of two resistors, a detector diode, and a compensator diode, and the input and the output of the diode detector 5 are connected to respective terminals of the detector diode in the d.c. path.
Another diode detector is described in U.S. Pat. No. 4,523,155 to Walczak et al. A detector diode and a compensator diode are provided, although the diodes are not serially connected. The detector diode is forward-biassed by a generator that includes the compensator diode, which is selected to provide temperature compensation.
U.S. Pat. No. 5,448,770 to Hietala et al. also describes a diode detector in which two diodes are not serially connected. The temperature coefficients of the diodes are compensated by a complex arrangement that generates the diodes' bias currents such that the currents have oppositely directed temperature coefficients.
These previous diode detectors suffer from various disadvantages, such as circuit complexity and/or imperfect temperature compensation. Accordingly, there is a need for a simpler diode detector that is better compensated than previous devices.